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Glacial-interglacial cycles (~100 kyr for the past 900 kyr) are driven by orbital eccentricity modulating precession and obliquity effects on insolation. Orbital forcing alone is weak (~0.1°C); ice-albedo, CO2, and ocean circulation feedbacks amplify orbital changes into ~10°C global temperature variations and ice-sheet extent oscillations.
From your study of Milankovitch cycles, you know that Earth's orbital parameters — eccentricity, obliquity, and precession — vary on timescales of tens to hundreds of thousands of years, changing the distribution of solar energy (insolation) across latitudes and seasons. The puzzle that glacial-interglacial cycles pose is one of amplification: orbital variations change global mean insolation by less than 0.1%, yet the climate system responds with temperature swings of ~10°C and ice sheets that advance and retreat across entire continents. The answer lies in powerful feedback mechanisms that multiply a small orbital nudge into a massive climate response.
The critical trigger is not total insolation but its distribution. Northern Hemisphere summer insolation at high latitudes (~65°N) is the key variable because it determines whether winter snowfall survives through summer. When obliquity is low and precession places Northern Hemisphere summer at aphelion (farthest from the Sun), summers are cool and short — snow persists, accumulates year over year, and ice sheets begin to grow. Once ice sheets form, the ice-albedo feedback kicks in: ice and snow reflect 60–90% of incoming solar radiation compared to 10–20% for bare ground or ocean. This cooling promotes more ice growth, which reflects more sunlight, which promotes more cooling — a self-reinforcing loop. Simultaneously, the cooling ocean absorbs more CO₂ from the atmosphere (cold water holds more dissolved gas), lowering atmospheric CO₂ concentrations and reducing the greenhouse effect, which amplifies cooling further.
The ~100,000-year periodicity that dominates ice age cycles over the past 900,000 years presents a famous puzzle. Eccentricity varies on this timescale, but its direct effect on insolation is the weakest of the three orbital parameters. The leading explanation is that eccentricity modulates the amplitude of precession: when eccentricity is near zero (a nearly circular orbit), precession has almost no effect on the seasonal distribution of insolation, so the triggers for ice sheet growth and collapse are muted. When eccentricity is high, precession swings produce large insolation contrasts between hemispheric summers, enabling the feedbacks described above to drive full glacial-interglacial transitions. The 100 kyr cycle thus emerges not from eccentricity's direct forcing but from its role as a gatekeeper that permits or suppresses the precession-driven feedbacks.
Terminations — the rapid transitions from glacial to interglacial conditions — are particularly dramatic. Deglaciation typically occurs in as little as 5,000–10,000 years, much faster than the slow buildup of ice sheets. This asymmetry reflects the nonlinear nature of the feedbacks: once ice sheets begin to retreat (triggered by increasing summer insolation), ice-albedo feedback accelerates warming, CO₂ rises as the warming ocean outgasses, and the combination drives further ice loss. Ice core records from Antarctica show that CO₂ and temperature rose nearly in lockstep during past deglaciations, with CO₂ sometimes lagging temperature by a few centuries — indicating that CO₂ acted as an amplifying feedback rather than the initial trigger, while still being essential to achieving the full magnitude of warming observed.